1. Technical Field of the Invention
The present invention relates generally to a radar apparatus designed to emit a radar wave modulated in frequency in time series and receive a return of the radar wave from a target through a plurality of receiving antennas to determine at least the azimuth or angular direction of the target.
2. Background Art
Recently, a radar is tried to be used in an anti-collision device of automotive vehicles. FM-CW (frequency-modulated continuous wave) radars designed to measure both the distance to and relative speed of a target are proposed for ease of miniaturization and reduction in manufacturing cost thereof.
Typical FM-CW radars transmit a radar signal Ss, as indicated by a solid line in FIG. 17(a), which is frequency-modulated with a triangular wave to have a frequency increased and decreased cyclically in a linear fashion and receive a radar return of the transmitted radar wave from a target. The received signal Sr, as indicated by a broken line, undergoes a delay of time Td the radar signal takes to travel from the radar to the target and back, that is, time depending upon the distance to the target, so that the received signal Sr is doppler-shifted in frequency by Fd depending upon the relative speed of the target. The received signal Sr and the transmitted signal Ss are mixed together by a mixer to produce a beat signal, as shown in FIG. 17(b), whose frequency is equal to a difference in frequency between the received signal Sr and the transmitted signal Sb. If the frequency of the beat signal when the frequency of the transmitted signal Ss is increased, which will be referred to below as a beat frequency in a modulated frequency-rising range, is defined as fu, the frequency of the beat signal when the frequency of the transmitted signal Ss is decreased, which will be referred to below as a beat frequency in a modulated frequency-falling range, is defined as fd, then distance R to and relative speed V of the target may be expressed as: ##EQU1##
where c is the propagation speed of a radio wave, T is a cycle of the triangular wave for modulation of the transmitted signal Ss, .DELTA.F is a variation in frequency of the transmitted signal Ss, and Fo is a central frequency of the transmitted signal Ss.
In use such an FM-CW radar in automotive vehicles, it is important to measure the azimuth or angular direction of a target as well as the distance R to and relative speed V of the target.
U.S. Pat. No. 5,369,409 to Urabe et al. teaches a FM-CW radar capable of measuring the azimuth of a target. The FM-CW radar consists of a plurality of antennas, a single transmitter, and a single receiver. The transmitter outputs a transmit signal to any one of the antennas for transmitting a radar wave. The antennas are so oriented in different directions, respectively, that beams emitted from adjacent two of the antennas may overlap with each other. The receiver receives one of signals received by the antennas to produce a beat signal. The radar switches combinations of the antennas used in transmitting the radar wave and receiving a return of the radar wave, in sequence, to obtain angular direction information about the target based on the strength of the beat signal and the location of the selected combination of the antennas.
Specifically, one or adjacent two of the antennas are selected for use both in transmitting the radar wave and receiving a return of the radar wave to detect the target within a range of the beam or a range where the beams overlap with each other. If the target is detected in a plurality of ranges, the angular direction of the target is determined based on strengths of signals received in the ranges.
The above conventional radar, however, has the drawback in that the switching of the combinations of the antennas to be used simultaneously is made every cycle of a change in frequency of the transmit signal, so that the azimuth of the target cannot be determined until the switching of all the combination is completed, thus requiring a long time until subsequent detection for updating the information about the target.
It is known that the strength of the beat signals are usually sensitive to various factors, and the resolution in range of the radar may be improved more greatly by measuring the azimuth using the phases of the beat signals than using the strength thereof. The above conventional radar, however, does not receive a return of the radar wave through a plurality of antennas simultaneously, and it is impossible to measure the azimuth of the target using the phases of the beat signals.
There is known a digital beam forming (DBF) technique for measuring the azimuth of a target using the phases of beat signals. The DBF is to form antenna patterns in the form of digital beams produced from a return of a radar wave received by a plurality of antennas simultaneously. Specifically, the DBF is equivalent to digitally performing functions of analog phase shifters installed in each radiating element in a conventional phased array system and of combining outputs of the analog phase shifters in an analog form.
The use of the DBF technique enables the strength of phase of beat signals to be measured in units of target angular directions each specified by one of the beams, thereby resulting in improved accuracy in measuring the azimuth of the target. The DBF technique has also the advantage that delaying each received signal is achieved in a phase shifter easily by software, thereby allowing a plurality of beam patterns to be formed simultaneously only by measuring signals received through the antennas for one cycle of changes in frequency of the transmit signal.
The realization of high accuracy measurement of the azimuth of a target using the DBF technique requires, as discussed above, simultaneous reception of a return of the same radar wave. This requires, as disclosed in Japanese Patent First Publication No. 5-150037, a plurality of expensive high-frequency receivers one for each antenna, thus resulting in increases in manufacturing cost and size of the system.